The present invention relates to a plasma display panel (PDP) and a phosphor adapted for use in various displays such as PDP.
The plasma display panel (PDP), which is adapted for preparation of a large screen and can be made thinner, attracts attention as a flat display panel which can be used in place of a cathode ray tube (CRT).
A surface discharge AC type PDP comprises a rear substrate provided with ribs defining discharge cells, address electrodes formed in the discharge cells and phosphor layers, and a front substrate provided with transparent electrodes extending in a direction perpendicular to the address electrodes, a transparent dielectric layer and a protective layer. The phosphor layer is formed by coating the discharge cells by means of, for example, a screen printing technique, with a paste of phosphor particles emitting red (R), green (G) or blue (B) lights, followed by drying the coating. Thickness of the phosphor layer is set at about 20 .mu.m. A mixed gas such as He--Xe or Ne--Xe is sealed as a discharge gas in the discharge cell formed between the rear substrate and the front substrate. In the PDP of this type, a surface discharge is brought about in the vicinity of the front substrate within the discharge cells. As a result, ultraviolet light is generated from the discharge gas sealed in the discharge cell, and the generated ultraviolet light excites the phosphor so as to permit the phosphor to emit a visible light.
It was customary in the past to form the phosphor layer of the PDP by using the phosphor prepared by the firing method using a flux. The phosphor particles prepared by this method are polyhedral. It should be noted that polyhedral phosphor particles are poor in dispersion capability. Therefore, if used for preparing a phosphor paste, the polyhedral phosphor particles tend to aggregate, giving rise to serious problems. For example, voids are generated within the phosphor layer, if a phosphor layer is formed by coating the paste. As a result, the thickness of the phosphor layer is increased so as to diminish the discharge space of the cell and, thus, to decrease the ultraviolet light serving to excite the phosphor. It follows that the brightness of the PDP is lowered. It should also be noted that, if a phosphor layer is formed by coating the paste containing polyhedral phosphors, the phosphor layer surface becomes irregular. Since the irregular surface of the phosphor layer brings about irregular reflection of light, the light loss is increased so as to lower the brightness. In addition, the brightness is rendered nonuniform over the entire region of the phosphor screen. As a matter of fact, the brightness of the PDP available on the market is 0.8 [lm/W] in terms of the entire efficiency in contrast to 1.7 [lm/W] for the CRT.
In order to increase the brightness of the PDP, various measures have been taken to date, including, for example the measures given below:
(1) Phosphor particles having a particle size of at most 1.5 .mu.m should be used for forming a phosphor layer; PA1 (2) The thickness of the phosphor layer should be gradually increased towards the front substrate; and PA1 (3) A white reflective layer consisting of titanium oxide should be formed below the phosphor layer.
However, any of these measures fails to essentially resolve the problem of the reduced brightness caused by the diminished discharge space resulting from the increased thickness of the phosphor layer and by the irregular reflection of light resulting from the irregular surface of the phosphor layer. Of course, it is of high importance to improve the brightness of the PDP.
It should also be noted that the conventional PDP is low in response speed, giving rise to a problem that an afterimage is generated in the case of displaying a moving picture. Particularly, a Zn.sub.2 SiO.sub.4 :Mn phosphor widely used as a green-emitting phosphor has a long life and, thus, is likely to generate a green afterimage. Naturally, it is also of high importance to suppress generation of an afterimage in the PDP.